Hollow-beam electron gun with a control electrode



July 9, 1968 ARNAUD ET AL 3,392,300

HOLLOW-BEAM ELECTRON GUN WITH A CONTROL ELECTRODE Filed Nov. .2, 1965 2 Sheets-Sheet 1 E PRIOR ART.

MIC/IE2 I'M flkw/wo P152125 V o/v Amemves July 9, 1968 ARNAUD ET AL 3,392,300

HOLLOW-BEAM ELECTRON GUN WITH A CONTROL ELECTRODE Filed Nov. 1965 2 Sheets-Sheet 2 MCI/EL M C. ARA/190.0 7 15 22s: W llvusu rams.

United States Patent 7 Claims. 61. s1s 34s ABSTRACT or THE DISCLOSURE Hollow-beam electron gun with a control electrode; the beam flows across an annular aperture provided with grid wires and through the aperture of an annularly apertured accelerating electrode. To eliminate spoke-like members bridging the gap in order to connect the inner and outer elements of :an electrode, the inner element of the control electrode is sustained by the grid wires only; while the inner element of the accelerating electrode, in the form of a disc, is supported by an insulating post extending centrally from the cathode and through a central hole of the inner control electrode element.

The present invention relates to electron beam tubes having hollo'w annular electron beams. The invention is particularly concerned with improvements in such tubes which have a control grid.

Hollow-beam electron beam tubes are primarily utilized at ultra high frequencies, such as in klystrons, and traveling wave tubes. Such tubes operating on the basis of interaction of fields, have often a substantial diameter. Traveling wave tubes, for example, have an interaction between the electron beam and a high frequency wave which propagates along a delay line. Such a delay line is often in the form of a metallic wire placed in helical form around an annular electron beam. Coupling between the high frequency field and the electrons of the annular beam is improved when the electrons are close to the spiral r metallic element of the delay line, and when the density J of electrons within the beam is high. For this reason it is preferred to utilize a beam having an annular cross-section, because of the high density of electrons which can be obtained at the region closest to the electrical field generated by the delay line.

Certain types of electron beam tubes having such an annular electron beam contain control grids. Ordinarily, such electron beam tubes have an annular cathode, an annular control grid, and an anode which is formed with an annular aperture aligned with the cathode and with the beam of electrons. A coil placed around the outside of the tube is usually present to create a magnetic field for focusing or control of the electron beam. If a grid is present, control of the beam can further be achieved by placing a 0 potential on the grid. The electronic beam emitted by the cathode can thus be closely controlled, particularly when the anode is supplied with a strong potential positive with respect to the cathode. The electrons which pass through the grid are accelerated towards the anode and directed towards the opening of the anode by the magnetic field of the coil, in order to form the electron-beam of annular cross section.

Mechanical supports for the grid structure and the anode structure interfere with the annular cross section of the tube and tend to distort the beam'and thus the proper interaction of the electric field along the delay line with an annular beam.

It is an object of the present invention to provide a tube in which the cross section of the electron beam is perfectly annular.

Briefly, in accordance with the present invention, the grid is formed of a pair of concentric, annular, substantially coplanar discs having a gap therebetween, with grid wire secured to the outer disc and extending inwardly to secure the inner disc and suspend the inner disc on the wires; a central stern is provided which passes through the center of the cathode and the inner, annular disc of the grid; the anode is formed of an annular outer disc and a center disc spaced from the inner periphery of the annular outer disc and secured to the stem passing through the cathode. The anode electrical terminal is brought out at the same end as the cathode end of the tube.

The structure, organization and operation of the invention will now be described more specifically in the following detailed description with reference to the accompanying drawings, in which:

FIG. 1 is a perspective, exploded view of a cathodegrid-anode structure of an annular beam tube according to the prior art;

FIG. 2 is a similar perspective, exploded view of a cathodegrid-anode structure according to the present invention, partly in section; and

FIG. 3 is a top view of an alternative embodiment of the grid structure.

The invention will best be understood by first explaining the operation of an annular beam tube, and the con struction thereof, in connection with an example from the prior art, and then explaining how the present invention solves the difiiculties previously experienced.

Referring now to FIG. 1, a cathode 1 has annular form; an emitting surface 2 is heated, for example, by a heater wire 3. A control electrode 4, and an anode structure 5 each are formed by a metal plate having an annular cutout 6, 7, respectively. The out-outs 6, 7, however, are not entirely circumferential; there would be nothing left to support the inner discs of the electrode. Thus, three (or more) metal cross elements 8, located at from each other support the central portion of the electrode 4, or of the anode 5. The elements themselves may best be manufactured by punching the annular gaps 6, 7 from solid metal discs, or the inner ortions of the electrodes may be secured to the outer rings by welded or brazed spokes 8. Grid wires 9 are welded, or brazed to the grid electrode 4, as shown. Ordinarily, these grid wires 9 are radially arranged in the region of the openings 6 between the central disc and the annular outer ring of the electrode 4. After manufacture of the elements themselves, they are assembled together closely spaced from each other within a tube structure not shown, and well known in the art.

During operation, the electron beam will not have an entirely annular shape but will be distorted by the presence of the spokes 8 connecting the inner and outer portions of the electrodes. The spokes of the grid and the anode are preferably aligned; yet, these structurally larger portions than the grid wire will intercept a part of the electron beam resulting in a power loss and heating of the anode and, further,-in a distortion of the beam. The beam, furthermore, will be split into three partial beam bundles, resulting in spurious oscillations within the tube, that is to say in variations in electron beam density and power along the axis of the tube. This has the effect that periodic variations within the tube are accentuated. In order to prevent undesirable interaction between the electron beam generated by the structure of FIG. 1 and a subsequent delay line, it is necessary that the diameter of the envelope,

and the diameter of the entire structure is greater than the theoretical length necessary if the bundle would be perfectly cylindrical. This greater diameter reduces the efficiency of interaction between the bundle of electron beams and the high frequency wave in a travelling wave tube. Further, an undesirable intersection of electrons by the spokes 8 may result when the grid becomes positive.

Referring now to FIG. 2, it will be seen that the spokes supporting the inner discs have been eliminated by the structure according to the invention. Further, the construction is simple and lends itself readily to assembly operations. Additionally, effects of distortion of the metallic elements themselves, due to heating, can be essentially eliminated by proper placement of the grid Wires, and the use of heat dissipating projections which can be applied to the structurally strong anode. The tube according to the present invention also has improved electro-optical qualities; it is sometimes of advantage to place the central portion of the anode in a plane slightly offset from the plane of the exterior portion of the anode, preferably closer to the cathode. The structure according to the present invention permits mounting of the central plate of the anode independently of the outer ring of the anode and permits adjustment of the plane of the central element with respect to the outer element of the anode, with great precision.

Referring now FIG. 2: the tube according to the present invention has, in essence, four principal parts: a cathode 11, a controlled electrode 12, an anode 13, and an insulating central stem 14. Cathode 11 is formed of an annular support 15, in U-cross section containing a heating coil 16 and consisting, for example, of a double spiral tungsten filament. One face of the cathode element is plane and is covered with an electron emitting material 17. The cathode structure further contains a pair of annular flanges 18, 19, as shown in FIG. 2.

The control electrode 12 is formed of an outer annular disc 20 and an inner annular disc 21. Both these discs are preferably of molybdenum. They are separated from each other by an annular gap 22. In accordance with the present invention, discs 20 and 21 are connected between each other by the grid wires themselves, for example goldplated molybdenum. Grid wires 23 are arranged approximately evenly spaced along the circumference and extending radially inwardly to form the grid in the gap 22. Each one of the grid wires 23 is secured to the disc 20 at one of its ends, and to the disc 21 at the other one. Preferably, the electrode assembly is manufactured in one step, by placing the discs 20, 21 and the grid wires 23 in a fixture, and welding or brazing all grid wires to the rings in one operation. This brazing, or soldering, is best done with gold. The electrode discs may also be gold plated, with the plating facing the cathode 11. Gold plating reduces secondary emission from the grid wires.

Anode 13 consists of an annular disc 24 and a circular disc 25 having a tubular projection 26 to improve heat dissipation. The units 24, 25 are of molybdenum; their adjacent edges are separated by an annular gap 27, of approximately the same dimension as the emissive part 17 of the cathode. Projection 26 may also be of molybdenum, and is provided to increase the surface of the disc 25.

The assembly of all the elements is done in this fashion: sub-assemblies 11, 12 and annular disc 24 of the anode 13 are placed in a fixture, spaced slightly from each other. Three ceramic rods, such as rods 28 shown separately, are placed in aligned holes 29 formed in the peripheral region of the elements. Thereafter, metal-ceramic seals are made in the region of the holes 29 and where the ceramic rods 28 pass. Thus, the ceramic rods 28 in holes 29 provide for strong and unvarying mechanical connection of all the electrodes, and at the same time provide the necessary electrical insulation. The center disc 25 of the anode is secured to a center stem 14 by means of a screw 30. Referring particularly to FIG. 2, disc 25 has a small downward projection, not specifically numbered for clarity of the drawing, which fits over a head 31 having a tapped Cir hole to receive attachment screw 30 and sealed on the center insulating stem 32 to provide for exact centering of the assembly. The lower portion of the stem 32, which is preferably of ceramic, is sealed to a base 33. Electrical connection 35 connects to head 31 and thus, by means of the small projection and attachment screw 30 provides electrical contact with anode disc 25. The base 33 is preferably formed with an offset, or shoulder 34 to provide for centering and seating of assembly 14 within the inner, annular flange 19 of the cathode assembly 11. The inner, and outer elements of the grid, as well as of the anode, are maintained with respect to each other in their predetermined position by rods 28 without having any structural support members interfering with the path of the electrons in the electron beam. Electrical connection between the two portions of the anode 13, that is between the annular disc 24 and the circular disc 25 can readily be effected outside the tube by means of connections to disc 24 and the central conductor 35.

FIG. 3 is a plan view of a modified form of a control grid which may be used in the assembly of FIG. 2'. The grid wires 23 are similar, that is to the grid wires of FIG. 2, but are not disposed radially; rather, they are located so as to have a tangential component with respect to the radial direction. In order to simplify the drawings, only a few of the grid wires 23 are shown, it being understood that similar wires are located around the circumference of annular rings 20, 21. The construction according to FIG. 3 has the advantage that the grid wires may deform in an axial direction due to thermal expansion, without causing dislocation or sagging of the inner grid assembly, because the arrangement is such that the inner ring will merely turn in its plane.

The construction according to the present invention thus provides an anode and grid assembly, each having a pair of elements with an annular gap therebetween, without any radial support members obstructing the free flow of electrons in a beam. The invention is not limited to travelling wave tubes or travelling wave delay lines, but may be utilized in connection with any type of beam tube in which an annular beam is desired.

We claim:

1. An electron beam tube having an annular cathode, an annular anode, and grid means located therebetween,

wherein said grid means comprises a pair of concentric, annular, substantially coplanar rings and an array of evenly spaced wires secured to the outer ring, extending inwardly and secured to the inner ring to suspend and support said inner ring within said outer ring;

a central stem passing through the center of said annular cathode and the inner ring of said grid means, and having means providing an electrical terminal at the end of said stem;

and wherein said annular anode comprises an annular ring in substantial registration with the outer one of said grid rings and a central anode plate secured to said central stern and connected to said electrical terminal;

and means securing the cathode, the outer grid ring and the outer anode ring in a predetermined longitudinally spaced position.

2. Tube as claimed in claim 1 wherein said anode plate is solid.

3. Tube as claimed in claim 1 including heat dissipating projections secured to said anode plate.

4. Tube as claimed in claim 1 said grid wires extending radially inwardly.

5. Tube as claimed in claim 1 said grid wires extending inwardly in a direction having a tangential component.

6. Tube as claimed in claim 1, wherein said grid wires are of gold-plated molybdenum.

7. Tube as claimed in claim 1 wherein the cathode is formed with an inwardly projecting flange, and said central stem is formed with a matching shoulder seating 5 6 against said flange to provide for exact centering of 2,547,061 4/1951 Touraton ct a1. 315--5.34 X said stem Within said tube. 2,796,548 6/ 1957 Becker 313-348 X 3,297,902 1/1967 Beggs 313-348 References Cited UNITED STATES PATENTS 5 JOHN W. HUCKERT, Primary Examiner. 2,261,154 11/1941 Hansen et a1. 313-343 JAMEiAssisll/"l Emmim- 2,489,156 11/1949 Rigrod 315-5.34 X 

